1. Field of the Invention
The invention relates generally to the field of magnetron sputtering and, more specifically, to planar magnetron sputtering apparatus for sputtering magnetic target materials using a rotating magnetic assembly.
2. Background of the Invention
Glow discharge sputtering is a well-known process that is widely used to deposit thin films of various kinds of ceramic and metallic materials onto the surfaces of objects. For example, glow discharge sputtering is commonly used in the electronics industry to produce integrated circuit semiconductors and photovoltaic cells, as well as the magnetic tapes and disks used in audio, video, and computer applications.
One type of glow discharge sputtering is diode sputtering. Diode sputtering is usually conducted in a vacuum chamber and in the presence of an inert sputtering gas, such as argon, that is maintained under very low pressure. The material to be sputtered (referred to as the target) is connected to the negative terminal of a DC power supply and serves as a cathode. The positive terminal of the power supply may be connected to a separate anode structure or to the vacuum chamber itself, depending on the application. The strong electric field between the target/cathode and the anode ionizes the sputtering gas, producing a glow discharge. Since the target/cathode is held at a strong negative potential, the positive ions from the glow discharge bombard the target material and eject target atoms, which then deposit on a workpiece or a substrate placed generally in line of sight of the target. Unfortunately, however, the diode sputtering process is slow and relatively inefficient compared to other film deposition techniques.
The efficiency of the diode sputtering process has been significantly increased by using a magnetic field to confine the glow discharge to the immediate vicinity of the target surface. Basically, while the sputtering yield (i.e., the number of target atoms dislodged or sputtered per incident ion) depends on the energies of the incident ions, the overall sputtering rate depends on both the energies of the incident ions as well as the total number of ions that bombard the target surface during a given time. Therefore, the sputtering rate can be increased by using a magnetic field to confine the ions and electrons produced in the glow discharge to the region immediately adjacent the surface of the target. The presence of such a plasma-confining magnetic field also has other benefits, such as allowing the sputtering operation to be conducted at lower gas pressures, confining the glow discharge to the neighborhood of the electrodes, and reducing electron bombardment of the substrates.
A common type of magnetic sputtering device is the planar magnetron, so named because the target is in the form of a flat circular or rectangular plate. Powerful magnets placed behind the target plate produce a strong plasma-confining magnetic field adjacent the front surface of the target, thus greatly increasing sputtering efficiency. While numerous shapes and configurations of plasma-confining magnetic fields exist, it is common to shape the plasma-confining magnetic field so that it forms a closed loop ring or "racetrack" over the surface of the target material. When viewed in cross-section, the flux lines of the magnetic field loop or arch over the surface-of the target, forming a magnetic "tunnel," which confines the glow discharge to a ring or racetrack shaped sputtering region proximate the front surface of the target. As is well-known, the electric field created by the high voltage between an anode and the target/cathode acting in combination with the closed loop magnetic field causes electrons within the glow discharge to gain a net velocity along the racetrack, with the magnitude and direction of the electron velocity vector being given by the vector cross product of the electric field vector E and the magnetic field vector B (known as the EXB velocity). The shape of the predominate electron path defines the portion of the target material that will be sputtered.
While planar magnetrons are widely used to sputter non-magnetic target materials, such as aluminum and its alloys, they have not proven particularly useful for sputtering magnetic materials such as, for example, iron, nickel, iron-nickel alloys, and cobalt-chromium alloys. Simply replacing a non-magnetic target in a planar magnetron with a ferromagnetic target of the same general configuration usually causes most, if not all, of the magnetic field to be shunted through the magnetic target. This reduces the intensity of the plasma-confining magnetic tunnel above the target to the point where it can no longer effectively confine the plasma over the surface of the target, thus reducing planar magnetron sputtering to that of ordinary diode sputtering with its attendant relatively slow sputter rate and inefficiency.
One solution to the problem of sputtering magnetic target materials has been to use very thin targets so that the target does not short the entire magnetic field. If the target is thin enough (approximately 2-3 mm for a highly ferrous material), sufficient excess magnetic flux will remain over the front surface of the target to produce a plasma-confining magnetic tunnel. Unfortunately, however, such thin targets are rapidly depleted, thus requiring frequent replacement and substantial downtime of the sputtering apparatus.
Another solution to the problem has been to strengthen the magnetic field so that it can saturate thicker targets, yet still produce a plasma-confining magnetic tunnel over the front surface of the target. Usually a field strength in the range of about 80-100 gauss in a direction parallel to the target surface is required to achieve the magnetron effect. While stronger magnets are more expensive, they can, at least theoretically, result in a magnetron suitable for sputtering magnetic targets of sufficient thicknesses to offset the additional cost of the stronger magnets. Unfortunately, however, sputtering magnetrons that magnetically saturate the target suffer from another problem that has proven much more difficult to overcome: namely, severe magnetic pinching of the plasma.
The magnetic pinching phenomenon is best understood by referring to FIG. 1 which illustrates, in schematic form, the forces acting on electrons in the region of the magnetic tunnel 106. In a prior art magnetron, a magnet assembly 100 produces a magnetic field that can be characterized by a plurality of magnetic flux lines 102 one of which is shown in FIG. 1. The flux line 102 shown in FIG. 1 is representative of the field shape immediately adjacent the front surface of the target. Points A and C are points in the central region of the tunnel, to the left and right of the central axis X respectively. Point B is a point on the central axis X of the tunnel, coincident with the flux line 102. The target locations within the tunnel fields are such that each of the points A, B, and C will be coincident with the front surface of the target at some time during the sputtering process. As is well-known, the curvature of the magnetic field 102 subjects electrons positioned at points A and C lateral forces (represented by arrows 104) that tend to push them toward the central axis X of the tunnel 106. Since the magnetic field has no vertical component (i.e., a component orthogonal to the plane of the pole piece) at point B, no lateral forces are exerted on electrons at point B. The action of the lateral forces on the electrons in the plasma tends to force or pinch them toward the central axis X of the magnetic tunnel 106. Since the erosion caused by sputtering is related to the density of the electrons in the glow discharge plasma, the effect of the pinching phenomenon increases the erosion rate along the central axis X of the magnetic tunnel 106.
While this pinching phenomenon occurs in all types of planar magnetrons with arched magnetic tunnels, the problem becomes much worse when sputtering magnetic target materials, as best seen in FIGS. 2(a)-(d). When sputtering magnetic target 200, a large portion of the magnetic flux 102 produced by the magnet assembly 202 will be shunted through the magnetic target 200. However, if the magnetic field is strong enough, sufficient magnetic flux will remain over the front surface of the target to produce an arched, plasma-confining magnetic tunnel 106. As was explained above, the pinching forces resulting from the arched magnetic tunnel will initially pinch the electrons toward the center of the tunnel, resulting in the greatest erosion rate at a point disposed central to the tunnel. However, as the target erodes, its ross-sectional area decreases, thus forcing additional magnetic flux from the target. Since the excess magnetic flux always takes the lowest energy path (i.e., the path of least resistance), it usually exits the target surface in the erosion area and re-enters the target just as soon as the cross-sectional area has increased to the point where the target material can again accommodate the excess flux. The liberated magnetic flux arching over the front surface of the target subjects electrons within the glow discharge plasma to even greater pinching forces, which substantially increases the electron density, thus erosion rate, along the center of the tunnel, as best seen in FIG. 2(c). As the target erodes further, more and more magnetic flux is liberated, resulting in stronger pinching forces, higher electron densities, and greater erosion rates. The result is a deep, spike-like erosion groove 204 in the target 200, as best seen in FIG. 2(d).
The fraction of the target material that has been sputtered away by the time the bottom of the erosion groove 204 reaches the back surface of the target is referred to as the target utilization, and is extremely low for most magnetic targets, in the range of 5%-15% at best. Since most target materials tend to be relatively expensive, such low target utilization is wasteful and increases the costs associated with the sputtering process. For example, although spent targets may be recycled and re-worked into new targets, the time spent changing and reworking targets can be significant and in any event, increases the overall cost of the sputtering operation.
One solution to improving the pinching phenomenon when using non-moving, or static, magnets is disclosed in U.S. Pat. No. 5,415,754 which discloses a magnet assembly positioned adjacent to the back surface of a target for generating a magnetic field having sufficient strength to magnetically saturate the target and to produce a plasma-confining magnetic tunnel over the front surface of the target, i.e., similar to the assembly shown in FIGS. 2(a)-(d). A magnetic shunt (shown in phantom as 250 in FIG. 2(d)) is positioned a distance from the back surface of the target and provides an alternate path for most of the excess magnetic flux liberated by the erosion of the target. The alternate path is characterized by lower magnetic resistance than the paths which exist at the front surface of the target and paths through the sputtering region.
Although a static magnetic assembly having a magnetic shunt provides improved target utilization, such static magnetic assemblies are only useful for use with relatively small targets. A relatively small target is only capable of deposition upon small object and small semiconductor wafers (e.g., a 150 mm diameter wafer). Consequently, the target assembly disclosed in U.S. Pat. No. 5,415,754 is not useful for deposition upon large semiconductor wafers (e.g., 200 mm diameter wafers). Recent advances to efficiently deposit magnetic materials onto large objects such as 200 mm or greater diameter semiconductor wafers using large target structures utilize a moving (rotating) magnetic assembly. Such moving magnet assemblies are disclosed in U.S. Pat. Nos. 4,444,643; 4,714,536; and 5,320,728. Each of these patents discloses a movable magnetic field source that is moved along the back of a target, parallel to the face of the target to produce a time-averaged sputtering of the target such that the uniformity of target utilization is increased and that the target is more efficiently sputtered. A significant development in moving magnet target utilization control systems is disclosed in U.S. Pat. No. 5,320,728 which discloses a structure having shaped pole pieces and a specific motion path that best utilizes the target material. However, although the shape of the pole piece and the path over the target are both optimized to produce an efficient sputtering of the target material, the pole pieces move in a defined path that will result in the pinching phenomenon. Although the pinching phenomenon is averaged over the entire target structure as the magnets move in their defined patterns, over time the pinching phenomenon will result in the target being eroded to have specific tracks in the target material, leaving significant non-sputtered, and thus unused, portions of the target.
Therefore, there is a need in the art for apparatus that reduces the pinching phenomenon in a system that uses a moving magnetic field generator within a target assembly of a magnetic material sputtering system.